A lithium ion secondary battery having a high energy density has recently received attention as a power source for portable electronic equipment, electric vehicles, and the like. A cylindrical lithium ion secondary battery, as shown in FIG. 13, includes a spirally wound, or rolled-up, electrode (2) housed in a positive electrode can (1), and a sealing plate (11) fixed on an opening of the positive electrode can (1). An insulator (12) is placed between the positive electrode can (1) and the sealing plate (11). A negative electrode terminal (13) having a built-in gas vent valve (14) is connected to the sealing plate (11).
The spirally wound electrode (2) comprises a negative electrode (21), separator (22) and positive electrode (23), each in the form of a strip. The negative electrode (21) and the positive electrode (23) are shifted widthwise of the separator when they are placed on the separator (22) to be spirally wound. An edge of the negative electrode (21) projects outwardly beyond an edge of separator (22) at one axial end of the spiral electrode (2), and an edge of the positive electrode (23) projects outwardly beyond an edge of the separator (22) at the other axial end of the spirally wound electrode (2). Current collectors (3) are placed on both ends of the spirally wound electrode (2). The current collector (3) on the negative electrode side is welded through a tab (31) onto a back of the sealing plate (11), and the current collector (3) on the positive electrode side is welded onto the bottom of the positive electrode can (1) through a tab (31). This structure makes it possible to take out electricity generated by the spirally wound electrode (2) via a negative electrode terminal (13) and the positive electrode can (1). The negative electrode terminal (13) is made of nickel, copper or stainless steel that is stable at a negative electrode potential. The positive electrode can (1) is made of aluminum or aluminum alloy that is stable at a positive electrode potential.
However, when conventional lithium ion secondary batteries, batteries A and B, are connected in series as shown in FIG. 14, for example, a negative electrode terminal (13b) made of nickel and a positive electrode can (1a) made of aluminum contact each other. There is a problem that the contacting of different metals for a long period causes electrical corrosion at the contacted portion and causes an increase in contact resistance.